1. Area of the Art
The present invention concerns the field of materials resistant to environmental extremes and in particular resistant to high radiation levels.
2. Description of the Prior Art
Nuclear energy and radioactive materials have posed seemingly insurmountable problems. There has been great public concern surrounding safety issues related to nuclear power plants, their design and operation. It appears that safe reactors are within the grasp of human engineering. The real problem posed may well be an environmental one caused by recycling and disposal of the spent nuclear fuels. Whether the spent fuels are reprocessed to yield additional fissionable material (the most efficient alternative from the view of long term energy needs) or whether the spent fuel is simply disposed of directly, there is a considerable volume of highly radioactive substances that must be isolated from the environment for long periods of time. The presently planned approach is the internment of the radioactive material in deep geologic formations where they can decay to a harmless level. Ideally these xe2x80x9cburiedxe2x80x9d wastes will remain environmentally isolated with no monitoring or human supervision. Unfortunately, one does not simply dump the wastes in a hole. These materials are constantly generating heat, and the emitted radiation alters and weakens most materials. This makes it difficult to even contain the materials, as the weakened containers are prone to breakage and leaking. Furthermore, potentially explosive gases, primarily hydrogen, are generated by interaction of radiation with many shielding materials. These problems impact both wastes and nuclear power plants. The safest possible design is to little avail if the structural elements of the power plant or the storage vessel deteriorate and/or experience hydrogen explosions.
In terms of waste the best present approach is to reduce the wastes to eliminate flammable solvents. The reduced wastes are then vitrified or otherwise converted into a stable form to prevent environmental migration. Generally, the reduced wastes (including spent fuel rods) are placed into a strong and resistant container for shipping and disposal. Ideally this container would show considerable shielding properties to facilitate transport and handling. In terms of nuclear power plants conventional shielding materials are often employed. The hope is to replace such materials or decommission the power plant before there is excess deterioration. Nevertheless, there remains the important task of producing special materials that display unusual resistance to radiation, heat and chemical conditions that generally accompany nuclear plants and radioactive wastes. Ideally, such materials have radiation shielding properties and can be used to shield and incase otherwise reduced wastes as well as decommissioned or damaged nuclear facilities.
The simplest and crudest of such materials is probably concrete. Because of the mineral inclusions in simple portland cement based materials or similar materials to which additional shielding materials (e.g. heavy metal particles) these substances provide shielding of nuclear radiation. However, simple concrete may not long survive under the severe chemical conditions provided by some nuclear facilities. In many applications the inherent brittleness of the concrete is a problem. When jarred or dropped, the material may develop cracks or leaks. Concrete tanks of liquid nuclear wastes have useful lifetimes of less than fifty years. Concrete is more effective against reduced vitrified wastes but is still far from ideal. There have also been a number of experiments with novel shielding-containment materials that would be easier to apply and have superior shielding and/or physical properties. The present inventor has disclosed such materials in U.S. Pat. No. 6,232,383. Although the material disclosed therein is a great advance over the prior art, it is not optimal in all aspects. The material shows tremendous tensile strength but is not ideal for applications where a certain amount of flexibility is desirable. Further, the disclosed formulae may not always show optimal resistance to radiation induced production of hydrogen (radiolysis).
The present invention is an improved nuclear shielding material that is initially flexible so as to effectively fill voids in radiation containment structures. The material is based on an amorphous organic matrix and is resistant to heat and radiation. Under very high temperatures the material is designed to undergo pyrolysis and transform into a strong ceramic material that retains the favorable radiation and hydrogen resistance of the original material.
As such the composition consists of uniform mixture of seven different component groups. The first component is a polymeric elastomer matrix such as a two part self-polymerizing system like RTF silicone rubber and constitutes about 10%-30% by weight of the final composition. The second component is a material to act as a gamma radiation shield, like tungsten carbide powder; the gamma shielding material makes up about 25%-75% by weight of the final composition. The third component is a neutron absorbing/gamma blocking material such as boron carbide powder and constitutes about 5%-10% by weight of the final composition. The fourth component is a heat conducting material such as diamond powder and makes up between about 0% and 5% by weight of the final composition. The fifth component is a high temperature resistant compound such as silicon dioxide powder which makes up between about between 0% and 5% by weight of the final composition. The sixth component is a second neutron absorbing compound which also imparts electrical conductivity, namely barium sulfate powder which makes up between 0% and 2% by weight of the final composition. Lastly, the seventh component is a hydrogen gas surpassing component which readily absorbs hydrogenxe2x80x94materials such as sponge palladium or other metals or intermetallic compoundsxe2x80x94and constitutes about 2-8% of the final composition.
The organic elastomer (first component) is preferably a two-part catalyst system so that all of the other components can be uniformly mixed together and then uniformly mixed into Part A of the RTF. Finally, Part B of the RTF is blended into the mixture which is then injected into its final location where it foams. polymerizes and hardens. Alternatively, other components can be uniformly blended into a mixture. Then part A and part B of the RTF can be uniformly blended and that mixture rapidly blended with the other component mixture and the resulting mixture injected into place before foam formation and polymerization heating has taken place.